Process for the manufacture of alumina (metallurgical grade) from highly silicious aluminous ores



ug. 22; 1944. Q GAN-HER 2,356,157

. PROCESS'FORA THE MANUFACTURE 0F ALUMINA. (METALLURGICAL GRADE) FROM HIGHLY sILIcIoUs ALUMINOUS oREs l Filed Feb. 3', 1942 l 1 Wa 2er v Patented Aug. 22, 1944 UNITED STATES PATENT OFFICE PROCESS FOR THE MANUFACTURE OF, ALU- MINA (METALLURGICAL GRADE) FROM HIGHLY SILICIOUS ALUMINOUS ORES Gant Gaither, Hopkinsville, Ky.

Application February 3, 1942, Serial No. 429,43

1 claim. (ci. :a3- 141) My discovery relates to the extraction of alumina by an acid process 4in which process the highly silicious content of the ore makes no material economical difference. In addition to use of the most eicacious acid in my process I have discovered a very economical method of forming the acid forcyclic use. The purification of alumina of contamination (iron and silica especially) is made Aquite satisfactorily :by recrystallization until it becomes metallurgical in grade.

In general terms, I take the aluminous ore, crush it to about 80 mesh and place it in suspension in a digestion tank provided with agitators to maintain suspension. In this tank the suspended crushed ore is leached with warm diluted hydrochloricacid. When this digestion is completed the contents of the digestion tank are drawn olf into a settling tank where the silicious residue settles to the bottom. In 'solution one nds the mixed chlorides of aluminum and iron with minor contamination. When settling has been completed the super-nant mixed. chloride solution is drawn off into a still for hydrochloric acid reformation. The residue is washed thoroughly to free of entrained mixed chlorides and hydrochloric acid. This Wash Water is then returned to first digestion tank for suspension of 'new aluminous ore for hydrochloric acid digestion, or the still. In the still the mixed chlorides are treated with a saturated aqueous solution of acid ammonium sulphate. By the application of heat suflcient to fboil the contents of the still, reaction occurs between the chemicals, producing hydrochloric acid which is distilled oil as a gas with steam, while iron sulphate, aluminum sulphate and ammonium sulphate are formed, mostly as alums. From this reaction thehvdrochloric acid is returned at will either vto first digestion tank to attack more laluminous material evaporated to proper Baum to produce crystallzation. These crystals are largely aluminum ammonium sulphate with fractional contaminations of iron and silica, this fraction however being too large at `this pointformetallurgical grade alumina. The mother liquor left in the rst crystallization tank, .together with that from the second crystallization tank,to be described below,is conducted to a precipitating tank where iron is eliminated completely by ammonia precipitation. Retumin-g to the rst ammonium alum crystals removed from the rst crystallizing pan we take these crystals, wash them (saving the Wash water) and redissolve in hot aqueous solution and lter. This solution is then placed in a second crystallizing pan. When these crystals are obtained they contain very much less of contaminating iron and silica, but usually are still too high for metallurgical grade. They are removed, put into solution, filtered and carried through a third crystallizin-g pan. These third crystals are suiliciently free of iron and silica to establish grade. This wash water is usually used as solvent for the ammonium bisulphate.

After the third crystallization the ammonia alum crystals are put into solution and conducted to a precipitation tank where they are treated with ammonia thus precipitating aluminum hydrate and leaving normal ammonium sulphate in solution. The aluminum hydrate is separated by filtration and taken to the calcining kiln where at about 1000 degrees centigrade it is calcined to pure -alumina (metallurgical grade). AThe solution of normal ammonium sulphate is evaporated to dryness and the ammonium sulphate is placed in a continuous furnace, operated by waste Vheat from the calcining furnace at about 300 degrees centigrade, one half the ammonia being driven off, and used in the various precipitation tanks, above described, and leaving acid ammonium sulphate formed in the kiln which is returned after solution to the hydrochloric acid still.

It will be noted that in this process wash waters are not discarded, but constantly used andwater is taken outof the cycle only by evaporation thus conserving all dissolved chemicals'. The two gases in the process, hydrochloric acid gas and amor to be absorbed to make hydrochloric acid in aqueous solution for storage.

The mixedalums thus' in solution in the still are then drawn into an evaporating pan and vmonia gas, are vfixed immediately after their generation. The hydrochloric vacid is xed either in aqueous solution or by action on aluminous ore, ammonia gas is xed ordinarily as anA ammonium sulphate, or as ammonium hydrate for storage.

It is understood that none of the chemical reactions or physical phenomena involved aboveV are I original, but it is my opinion that the sequences and results are productive of metallurgical grade alumina, and have never before been thus discovered.l

The chemical equations involved in the above are as follows:

ammonium acid sulfate to the solution and heating' to drive 01T, hydrochloric acid which is recovered andre-used in digesting more ore. and obtaining a residual solution of mixed sulfates of a1u'minum,iron, and ammonium. crystallizing ammonium alum Ifrom said solution, purifying the ammonium alum to the desired degree vby recrystallization; dissolving the recrystallized ammoniumA alum, precipitating aluminum hydroxide therefrom by introducing ammonia,;

liquor from the crystallization of alum and precipitating iron therefrom, leaving a solution containing ammonium sulphate, and heating said solution to recover ammonium acid sulphate and ammonia. therefrom for re-use in the process.-

GANT GAI'IHER.

separating the aluminum hydroxide product from the solution, collecting this solution and the mother 

